Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in molecular systems. In a Raman spectroscopic experiment, a monochromatic beam of light of a particular wavelength range passes through a sample of molecules and a spectrum of scattered light is emitted. The term “light” is not limited to electromagnetic radiation with wavelengths that lie in the visible portion of the electromagnetic spectrum but also includes electromagnetic radiation with wavelengths outside the visible portion, such as the infrared and ultraviolet portions of the electromagnetic spectrum, and can be used to refer to both classical and quantum electromagnetic radiation. The spectrum of wavelengths emitted from the molecule is called a “Raman spectrum” and the emitted light is called “Raman scattered light.” A Raman spectrum can reveal electronic, vibrational, and rotational energies levels of a molecule. Different molecules produce different Raman spectrums that can be used like a fingerprint to identify molecules and even determine the structure of molecules.
The Raman scattered light generated by a compound (or ion) adsorbed on or within a few nanometers of a structured metal surface can be 103-1012 times greater than the Raman scattered light generated by the same compound in solution or in the gas phase. This process of analyzing a compound is called surface-enhanced Raman spectroscopy (“SERS”). In recent years, SERS has emerged as a routine and powerful tool for investigating molecular structures and characterizing interfacial and thin-film systems, and even enables single-molecule detection. Engineers, physicists, and chemists continue to seek improvements in systems and methods for performing SERS.